The invention pertains to cementing with a gas-containing cement slurry, more particularly, it pertains to cementing a void in a subterranean formation. Commonly such voids are created or encountered in the drilling of boreholes in the production of oil or gas or of geothermal fluids from the earth. The invention is especially adapted to the use of "foamed" cements in the completion of such wells at great depths or in weak subterranean formations which are easily fractured by cement slurries of ordinary weights.
Traditionally, cement slurries for use in such applications have been prepared by blending dry cement and additives with water and liquid additives in mixing tanks employing mechanical agitation to achieve relatively homogeneous slurries. Aerated or gasified (i.e. "foamed") cement slurries have been prepared for surface applications by addition of a foaming agent or air entraining agent to the mixing tank. For use in subterranean foamed cement applications, the cement slurry has been prepared in the traditional fashion, a foaming agent has been subsequently added to the slurry at a point downstream from the mixing tank, and air (or gas such as nitrogen) has then been added to the slurry at a point further downstream prior to introduction of the slurry into the subterranean formation. Mixing of the gas so added has been achieved through the turbulence created by the flow of the slurry in the conduit or from the energy of the gas itself. Such turbulence has been created by the injection of air, under pressure, at an angle substantially normal to the flow of cement slurry in the conduit through a "tee" or a "y" in the conduit; British Pat. 819,229.
However, these methods of adding a gas to a cement slurry have not always resulted in a uniform mixture of gas and slurry. When a well is being cemented which does not have a positive backpressure, the foamed cement prepared may not be a uniform mixture of gas and slurry since accurate regulation of the liquid and gaseous components is difficult to achieve.
Chokes have been widely used in the oilfield to control the flow rate of high pressure fluids issuing from wells. These chokes are usually centered in a flow line so that the fluid's velocity decays to a point where the fluid emitted no longer erodes the wall of the piping. A system of opposed chokes, similar to that described herein, has been used to depressurize oilfield fluids, as described in copending U.S. application Ser. No. 185,087 filed Sept. 8, 1980, by Warren M. Zingg et al.